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DSP Chip Selection
The design for the
digital demodulator has been developed through the use
of DSP technology. The team has previous experience with
microcontrollers, but they were found to be unfeasible
in a previous solution attempt. No member of our group
had any previous experience with DSP hardware. A lot of
research was needed to understand how they work.
Research began by going
to websites of two of the largest manufacturers of DSP
chips, Analog Devices and Texas Instruments. According
to their websites, the DSP chips from TI appeared to be
cheaper and take up less board space. Both sites
contained information on how DSP chips work and their
specifications. Each company contained several different
platforms, or groups of DSP chips. Because the task that
the analog circuit is performing is very trivial to a
DSP chip, all DSP chips are capable of implementing the
circuit. However, there are many different aspects to
consider when selecting a chip.
The C2000 platform is
the best to use because they are the cheapest and take
up the least board space. Since we need a resolution
greater than 8 bits, only the chips with 10 bits and 12
bits resolution were considered. To pick a chip, we came
up with the following table to compare several of the
chips from TI. The chips all have built in
analog-to-digital converters at their inputs. The number
of these ADCs limits how many of the circuits you can
implement with each chip. Since the analog circuit
inputs two analog signals, the number of circuits for
each chip can be calculated by simply dividing the
number of ADCs by two. The board area and cost of each
DSP chip were taken from the data sheet.
|
DSP Chip |
Bits |
ADC inputs |
Outputs |
# of
circuits |
BA (mm^2) |
Cost |
BA/Circuit
(mm^2) |
Cost/Circuit |
|
LF2401a |
10 |
5 |
13 |
2 |
81 |
$3.49 |
40.50 |
$1.75 |
|
LF2402a |
10 |
8 |
40 |
4 |
417.12 |
$7.09 |
104.28 |
$1.77 |
|
LF2407a |
10 |
16 |
40 |
8 |
484 |
$8.83 |
60.50 |
$1.10 |
|
F241 |
10 |
8 |
26 |
4 |
632.27 |
$12.37 |
158.07 |
$3.09 |
|
F2801 |
12 |
16 |
35 |
8 |
100 |
$5.79 |
12.50 |
$0.72 |
|
F2806 |
12 |
16 |
35 |
8 |
100 |
$8.69 |
12.50 |
$1.09 |
|
F2808 |
12 |
16 |
35 |
8 |
100 |
$11.52 |
12.50 |
$1.44 |
Table 1: DSP Chip
Comparison
Based on these results,
we decided that the 2801 was the best chip for this
task. It is the best chip in terms of cost per circuit
and is tied with the other 280x chips for smallest board
space per circuit. It is smaller than the other chips
because it offers a plastic ball grid array package
option. This means that the connections are underneath
the chip. This saves board space compared to having the
pins come out on each side of the chip, like an op-amp.
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